Today, sensors provided as amperometric sensors or enzyme electrodes or as optical sensors (optodes) are widely used to determine the amount of or to detect certain substances such as glucose, oxygen, CO.sub.2, in the blood and in other body liquids.
The construction and the function of an amperometric enzyme electrode is known for example from EP-A 0 603 154 of the applicant. The construction and the function of an optode is known for instance from Biosensors & Bioelectronics 5(1990) pp 137-148.
Amperometric sensors for the determination of glucose, lactate or creatinine are preferably constructed with oxidoreductases and a detection device to determine hydrogen peroxide (base electrode). They function in such a way that the oxidase such as glucose oxidase, lactate oxidase and sarcosin oxidase oxidizes the analyte to be determined with oxygen to the correspondent oxidation product and hydrogen peroxide, the concentration of hydrogen peroxide produced being proportional to the concentration of the analyte and being measured by anodic oxidation at approximately 650 mV versus Ag/AgCl at stainless steel or carbon electrodes (base electrodes). Alternatively, measuring may also be carried out at electrodes having a catalytical effect such as platinized carbon and mangane dioxide, at a reduced oxidation voltage (approximately 300 mV).
Further possibilities of determination are measuring the oxygen consumption or using mediators which may be measured by oxidation at the electrode instead of the hydrogen peroxide and also serve, in an oxidized form, as a substitute for oxygen.
The classical amperometric sensor comprises four layers: a base electrode, an interference-blocking layer, an enzyme layer and a cover membrane or cover layer respectively. This construction is schematically shown in the attached FIG. 1, reference number 1 indicating an electroconductive layer, e.g. from silver, applied onto a support (not shown). The base electrode 2 is applied onto the electroconductive layer 1. On top of the base electrode 2, the interference-blocking layer 3, the enzyme layer 4 and the cover membrane 5 are provided. The cover membrane 5 is in contact with the sample.
The interference-blocking layer 3 of an amperometric sensor serves to keep away from the electrode any electroactive substances of the sample such as paracetamole, uric acid, ascorbic acid, which may be oxidized directly at the electrode surface and thus show wrong, excessive signals. As the interference-blocking layer, layers from cellulose acetate and polyphenylene diamine are of preferred use, the polyphenylene diamine layer being produced by polymerisation of phenylene diamine directly onto the base electrode. Unplasticised polyvinyl chloride (PVC) and nafion may also be applied directly from their solution.
As the cover layer 5, a polycarbonate membrane made microporous by etching is frequently used, the pore width typically being 0.03.mu.m at a porosity of less than 5%. This cover membrane is biocompatible and limits diffusion due to its reduced porosity, but does not prevent enzymes from being transported through the pores. To improve the diffusion properties, this layer is frequently laminated and/or additionally coated. By wetting with hydrophobe plasticisers, a so-called supported liquid membrane may be prepared.
A cover layer 5 of a porous polycarbonate membrane has the drawback of being incapable of sufficiently protecting the underlying enzyme layer 4 from proteases. Moreover, it is incapable of preventing the washing out of enzymes from the enzyme layer 4, since enzymes are capable of diffusing through pores 0.03 .mu.m wide.
Cover layers are often mechanically attached to the enzyme layer. When the cover layer is combined with the enzyme layer, this layer has to be mechanically attached to the base electrode. Such mechanical attachments are expensive, technically complex and frequently cause problems insofar as it is difficult to apply the membrane onto the underlying layer without producing air bubbles. This usually restricts the constructive freedom when designing sensors, since an embossed electrode surface is required to attach a membrane under mechanical tension to the electrode. The tension required frequently causes fissures and creases. Additionally, sheet membranes are relatively thick, and therefore the sensors produced have comparatively low electric currents and long response times.
To prepare the cover membrane it is further known to apply a solution of the polymer onto the enzyme layer and to evaporate the solvent. Thus for example, cover membranes of nafion, PVC, polyurethane, silicone, polyacrylate (p-HEMA) and cellulose acetate which stick to the underlying layer without any mechanical attachment, i.e. only by adhesion, may be prepared.
The polymers used so far which may be applied directly from their solution onto the enzyme layer or the electrode include only a few, such as nafion and cellulose acetate, which are selective towards electrochemically active interferents. Additionally, many polymers are soluble only in volatile, aggressive or toxic solvents, such as cellulose acetate in DMSO and acetone, and PVC in tetrahydrofurane and cyclohexanone. This circumstance is relevant for the production process as well as for safety reasons. It is also relevant for the electrode itself, since plastic portions may be deteriorated or enzymes present in the enzyme layer may be destroyed by these solvents. From plasticised PVC, plasticisers may diffuse into surrounding plastic portions or into the enzyme layer.
Another drawback consists in that in most polymers, particularly in plasticised PVC, permeability to the analytes such as glucose or lactate may be adjusted only by varying the layer thickness, since permeability is primarily due to faults in the membrane and the porosity thus produced. Even slight differences in the layer thickness may cause a total loss in permeability.